[opt] Cleanups related to legacy PM deprecation

Remove dead code related to "FPasses". This was a leftover from
commit 7a5332b9.

Do not mention -enable-new-pm in error messages. The option does
not exist any longer.

Remove the addPass helper. Only one use remained, so we can just
"inline" it manually to keep the code related to legacy PM a bit
less spread out.

Differential Revision: https://reviews.llvm.org/D148082